<!-- <!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>demo</title>
	<style type="text/css">
        .test.test2{
            background-COLOR:#a00;
            width:100px;
            height: 100px;
        }
        .test.test2{
            background-COLOR:#0e0;
            width:100px;
            height: 100px;
        }

    </style>
</head>
<body>
	<div id="tet" class="test test2"></div>
</body>
</html> -->

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>demo</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        html, body {
            height: 100%;
        }
        #app {
            height: 100%;
            width: 100%;
            background-color: #cff;
            align-items: center;
            justify-content: center;
            display: flex;
        }
        #container {
            background-color: #fff;
            flex: 1;
            width: 80%;
            height: 0;
            padding-bottom: 150%;
        }

    </style>
</head>
<body>
    <div id="app">
        <div id="container"></div>
    </div>
</body>
</html>